Plant growth regulators are useful for influencing a range of plant developmental processes including stem elongation, germination, dormancy, flowering, sex expression, enzyme induction, fruit size and quality, as well as leaf and fruit senescence. Plant growth regulators may be formulated in at least five different types of formulations: 1) solutions, 2) wettable powders, 3) soluble powders, 4) tablets, and 5) water-soluble or dispersible granules. In order to use such formulations, they must be diluted in an aqueous solution prior to conventional spray application. Each of the conventional types of formulations has disadvantages, so research to provide improved plant growth regulator formulations continues. The disadvantages of the conventional formulations will be discussed with reference to two important plant growth regulator classes, the gibberellins and the cytokinins, as representative of conventional formulations of plant growth regulators in general.
Gibberellins are one class of plant growth regulators which are diterpenoid acids. Gibberellins are commercially produced by fermentation of a natural fungus, Gibberella fugikuroi. Gibberellins are marketed under various trade names and are commercially used on a variety of fruit orchards, vegetable crops, row crops, and ornamental crops. The predominantly used gibberellic acid is GA3, formulated in any of the five forms described above. The other commonly used gibberellins are a combination of two, (GA4+7) which are primarily formulated as solutions in isopropyl alcohol, tetrahydrofurfuryl alcohol (THFA) or propylene glycol. Due to diterpenoid structure of gibberellins containing double bonds and cyclic lactone, they are less stable in liquid form, especially in water.
Cytokinins are another class of plant growth regulators which are generally defined as N6-substituted adenine derivatives such as trans-zeatin, 6-benzyladenine and kinetin. Recently, a new class of cytokinins has been identified and they possess N-phenylurea substituted structure such as forchlorfenuron (CPPU) and thidiazuron (TDZ). Cytokinins are of extreme importance in regulating plant growth and development, especially cell division. They are marketed under various trade names and are commercially used in fruit thinning and sizing as well as pre- and post-harvest treatments of ornamental plants and flowers. Cytokinins have low solubility in water or solvent and solubility improvement in formulation and in use dilution are important for obtaining their biological effectiveness.